The invention relates generally to a method for water jet stripping and recontouring coated articles of manufacture and more specifically to a method for water jet stripping and recontouring coated buckets and blades for gas turbines.
Gas turbine buckets and blades often experience cracking on the surface of the parts. These cracks may extend into the “base-metal” or casting to significant depths. Many times, this cracking is caused when cracks initiate in a brittle or non-ductile coating and then continue to propagate into the “base-metal” or casting to significant depths. In order to affect a repair, it is common that the cracks must be removed prior to continued engine service. Typically, this repair involves removing the coating by chemical or mechanical means and then mechanically removing material, usually through manual grinding or computer numerical control (CNC) machining, until the cracks have been eliminated. However, the problem of cracks extending through an outer coating and into a base metal is not limited to turbine buckets and blades, or turbine parts in general, but extend to a broad range of articles of manufacture.
Cracks found on the surface of gas turbine hardware after engine operation, especially buckets, often must be removed as part of the repair process. This removal of these cracks is typically achieved by grinding (blending) away metal on the surface of the part until the cracks have been removed. This material removal is typically done by manual grinding, which is very labor intensive, or by CNC machining, which is extremely time consuming. Manual grinding for defect removal may result in the creation of uncontrolled shapes that do not conform with the original design profile of the article.
Miller et al. (U.S. Pat. No. 6,905,396) provides a method of removing a coating from a substrate with a pressurized abrasive water jet while moving the abrasive water jet and/or the component relative to one another to fully expose the coating to the fluid stream. Hashish et al. (U.S. Pat. No. 5,704,824) describes a method and apparatus for milling objects by means of a high velocity abrasive water jet. Control means are provided that allow uniform and variable depth of milling of complex shapes and automatic variation in relative speed, standoff distance, angle and pressure. Miller et al. and Hashish et al. do not address a one-setup process for removing the coating and the surface defect from an article of manufacture and recontouring the surface.
Accordingly, there is a need to provide a method for removal of cracks and surface imperfections in the base metal of these articles of manufacture, which have an outer coating, and to perform the process in one setup. Further is a need to maintain the contoured shape of the original article without deforming the shape as often occurs in a manual repair. Still further, there exists a need to perform the process quickly and at a low cost.